1. Field of the Invention
The present invention relates to a tetrahydroxy compound. In addition, the present invention also relates to a charge transport material for use in organic semiconductor devices using a charge transport material such as organic photoreceptors, organic light emitting devices, organic thin film transistors, and organic solar cells. In addition, the present invention also relates to a method for preparing the tetrahydroxy compound.
2. Discussion of the Background
Inorganic semiconductor devices are typically prepared by a vacuum film forming method. In contrast, organic semiconductor devices can be prepared by a coating method. Therefore, organic semiconductor devices have the following advantages over inorganic semiconductor devices
(1) devices having a large area can be easily prepared;
(2) the devices have relatively low manufacturing costs; and
(3) the devices have good on-demand property.
Therefore, organic semiconductor devices have been actively investigated.
On the other hand, the structure of semiconductor devices have been complicated more and more and recent semiconductor devices typically have a plurality of layers to improve the functions and performances thereof. For example, organic electrophotographic photoreceptors typically have a three layer structure such that an undercoat layer, a charge generation layer, and a charge transport layer are overlaid on an electroconductive substrate (such as aluminum drums). In addition, a photoreceptor in which a protective layer is further formed on a charge transport layer as an outermost layer so that the photoreceptor has a high performance, is disclosed in published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 2001-235889.
Organic light emitting devices typically have a layered structure such that plural layers such as a hole injection layer, a hole transfer layer, a light emitting layer and an electron transport layer are overlaid. Techniques such that the above-mentioned layers are formed by a coating method using an inkjet method have been proposed. In this case, it is necessary for a coating layer not to dissolve the layer on which the coating liquid is coated, i.e., it is necessary for the coating liquid not to include a solvent dissolving the layer on which the coating liquid is coated.
Thus, when a device in which an organic layer is formed on another organic layer by a coating method, the materials included in the coating liquids or the upper and lower organic layers preferably have different solvent-solubilities to prepare a device in which the upper and lower layers have clear interface.
On the other hand, a variety of charge transport materials have been proposed. Such charge transport materials typically have a formula including a benzene ring. Therefore, the variety of charge transport materials have similar solvent solubilities, and are soluble in organic solvents such as toluene, ethyl acetate and tetrahydrofuran. Almost all of such charge transport materials are hardly soluble in lower alcohol solvents such as methanol and ethanol. Therefore, if a charge transport layer coating liquid mainly including such an alcohol solvent is coated on such charge transport materials, a layered charge transport layer having a clear interface therein can be prepared. However, charge transport materials which can be well dissolved in a lower alcohol solvent have hardly be found.
JP-A2000-23589 mentioned above proposes a photoreceptor in which an alcohol soluble charge transport material is used as one component of the outermost layer thereof. It is described therein that the alcohol soluble charge transport material is compounds having a benzadine structure or a triphenylamine structure into which a carboxyl group or a hydroxy group is incorporated. In addition, it is described therein that N,N′-diphenyl-N,N′-bis(3-hydroxyphenyl)-[1,1′-biphenyl]-4,4′-diamine and another compound are used as an alcohol soluble charge transport material. However, as a result of the present inventors' study, it is found that the solubility of the compounds to methanol is insufficient. This is because the molecular weight of the compounds per one hydroxyl group is still high and therefore the compounds have low polarity.
Therefore, it is considered to be preferable that three or four hydroxyl groups are incorporated into such a compound. However, it is difficult to incorporate three or four hydroxyl groups into such a compound because the manufacturing costs of the compound seriously increase and in addition the charge transport property thereof deteriorates. Namely, a charge transport material having a good combination of charge transport property and alcohol solvent solubility has not yet been developed.
JP-A2002-6517 discloses an organic photoreceptor having a protective layer serving as an outermost layer in which an alcohol soluble charge transport material such as charge transport materials modified by a hydroxyalkyl group or a hydroxyalkoxyl group is included as one component. Specific examples of the alcohol soluble charge transport material are N,N-bis(4-hydroxyethylphenyl)-4-ethyl-3-methylaniline and three other compounds, each of which includes a 2-hydroxyethyl group as a substituent. In order to synthesis such compounds, ethylene oxide is typically used. Since ethylene oxide is a dangerous compound, it is not easy to synthesize the compounds.
JP-A2003-76043 discloses an organic photoreceptor having a protective layer serving as an outermost layer in which an alcohol soluble charge transport material such as compounds having one or two polyalkylene oxide groups as the end hydroxyl group is included as one component. Specific examples of the alcohol soluble charge transport material are 1,1-bis(4-hydroxyphenyl)-1-[4-(paraditolylamino)phenyl]ethane and five other compounds. However, as a result of the present inventors' study, it is found that the solubility of the compounds to methanol is insufficient. This is because the molecular weight of the compounds per one hydroxyl group is still high.
Because of these reasons, a need exists for an alcohol soluble charge transport material having a good charge transport property, a good solubility to lower alcohols such that the resultant alcohol solution has a solid content not lower than 20% by weight, and low manufacturing costs.